Process for the preparation of linear olefins and use thereof to prepare linear alcohols

ABSTRACT

Process for the preparation of a mixture comprising C 5 + linear olefins, which process comprises the steps of 
     (a) reacting carbon monoxide and hydrogen in the presence of an effective amount of Fischer-Tropsch catalyst under Fischer-Tropsch reaction conditions; 
     (b) separating from the hydrocarbon mixture thus prepared at least one hydrocarbon fraction, of which at least 95% by weight consists of hydrocarbons containing 15 carbon atoms or more; 
     (c) contacting this hydrocarbon fraction with hydrogen in the presence of an effective amount of hydrogenation catalyst under hydrogenation conditions; 
     (d) subjecting the hydrogenated hydrocarbon fraction thus obtained to a mild thermal cracking treatment; and 
     (e) separating from the cracked product thus prepared the mixture comprising C 5 + linear olefins.

The present invention relates to a process for the preparation of linearolefins and to a process to prepare linear alcohols from anolefin-containing feed, which is at least partly based on these linearolefins.

BACKGROUND OF THE INVENTION

There are various methods known in the art to prepare linear olefins.

Such process is disclosed in U.S. Pat. No. 4,579,986. This U.S. patentdiscloses a process for the preparation of linear C₁₀-C₂₀ olefins, whichprocess comprises preparing a mixture of hydrocarbons substantiallyconsisting of linear paraffins by:

(a) contacting a mixture of carbon monoxide and hydrogen at elevatedtemperature and pressure with a cobalt-containing catalyst,

(b) separating from the paraffin mixture thus prepared a heavy fractionwhich consists substantially of C₂₀+ paraffins, and

(c) converting at least this heavy fraction (a “wax”) by mild thermalcracking into a mixture of hydrocarbons which consists substantially oflinear olefins and contains the desired C₁₀-C₂₀ olefins.

Although the wax cracking method according to U.S. Pat. No. 4,579,986performs satisfactorily, there is still room for improvement.Particularly if the starting point is to produce an olefin-containingfeed which can be used as (part of) the feedstock for a hydroformylationreaction stage to produce linear detergent and plasticizer alcohols, themethod according to U.S. Pat. No. 4,579,986 can be improved. Namely,linear plasticizer alcohols typically contain from 7 to 11 carbon atoms,while linear detergent alcohols typically contain 12 to 15 carbon atoms.Accordingly, any hydrocarbon fraction produced to serve at least partlyas the source of hydroformylation feedstocks should contain asignificant portion of C₆ to C₁₄ olefins, at least 80% by weight, butpreferably at least 85% by weight, of which consists of thecorresponding linear α-olefins. It was found that by hydrogenating thewax feed prior to subjecting it to the mild thermal cracking treatmentvery high quality C₆ to C₁₀ and C₁₁ to C₁₄ linear α-olefins areproduced: the C₆ to C₁₄ olefins produced (contained in a mixture of C₅+olefins) consist for more than 80% by weight of C₆ to C₁₄ linearα-olefins.

SUMMARY OF THE INVENTION

A process for the preparation of a mixture comprising C₅+ linearolefins, which process comprises the steps of

(a) reacting carbon monoxide and hydrogen in the presence of aFischer-Tropsch catalyst under Fischer-Tropsch reaction conditionsthereby producing a hydrocarbon mixture;

(b) separating, from the hydrocarbon mixture, at least one hydrocarbonfraction, of which at least 95% by weight consists of hydrocarbonscontaining 15 carbon atoms or more;

(c) contacting the thus-separated hydrocarbon fraction with hydrogen inthe presence of a hydrogenation catalyst under hydrogenation conditionsthereby producing a hydrogenated hydrocarbon fraction;

(d) subjecting said hydrogenated hydrocarbon fraction to a mild thermalcracking treatment thereby producing a cracked product; and

(e) separating, from the cracked product, a mixture comprising C₅+linear olefins.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to the preparation of linear olefins by aprocess which also involves a Fischer-Tropsch hydrocarbon synthesisreaction.

Accordingly, in a first aspect the present invention relates to aprocess for the preparation of a mixture comprising C₅+ linear olefins,which process comprises the steps of

(a) reacting carbon monoxide and hydrogen in the presence of aneffective amount of Fischer-Tropsch catalyst under Fischer-Tropschreaction conditions;

(b) separating from the hydrocarbon mixture thus prepared at least onehydrocarbon fraction, of which at least 95% by weight consists ofhydrocarbons containing 15 carbon atoms or more;

(c) contacting this hydrocarbon fraction with hydrogen in the presenceof an effective amount of hydrogenation catalyst under hydrogenationconditions;

(d) subjecting the hydrogenated hydrocarbon fraction thus obtained to amild thermal cracking treatment; and

(e) separating from the cracked product thus prepared the mixturecomprising C₅+ linear olefins.

The product mixture comprising C₅+ linear olefins preferably is amixture comprising C₅ to C_(m) linear olefins with m being an integer offrom 10 to 20, preferably 12 to 18, more preferably 12 to 15. A veryuseful mixture is a mixture comprising C₅ to C₁₄ linear olefins. Suchmixture suitably comprises at least 20% by weight, and more preferablyfrom 25 to 50% by weight, of C₁₁ to C₁₄ linear α-olefins. The C₅ to C₁₀linear α-olefins typically constitute up to 75% by weight of the stream,suitably from 40 to 75% by weight. The balance up to 100% by weight,which forms a relatively small proportion of the stream, consists ofhydrocarbons other than the olefins mentioned, such as C₄ hydrocarbonsand the corresponding C₅+ linear alkanes, iso-alkanes, iso-olefins,internal olefins and dienes. Typically this small proportion of otherhydrocarbons will not exceed 20% by weight and suitably is less than 10%by weight.

In step (a) of the present process hydrocarbons are formed by reactingcarbon monoxide and hydrogen under suitable conditions. In general, thepreparation of hydrocarbons from a mixture of carbon monoxide andhydrogen at elevated temperature and pressure in the presence of aneffective amount of a suitable catalyst is known as the Fischer-Tropschhydrocarbon synthesis. Catalysts used in this hydrocarbon synthesis arenormally referred to as Fischer-Tropsch catalysts and usually compriseone or more metals from Groups 8, 9 and 10 of the Periodic Table ofElements, optionally together with one or more promoters, and a carriermaterial. In particular, iron, nickel, cobalt and ruthenium are wellknown catalytically active metals for such catalyst. The Fischer-Tropschcatalyst to be used in step (a) of the present process suitablycomprises a porous carrier, in particular a refractory oxide carrier.Examples of suitable refractory oxide carriers include alumina, silica,titania, zirconia or mixtures thereof, such as silica-alumina orphysical mixtures such as silica and titania. Very suitable carriers arethose comprising titania, zirconia or mixtures thereof. Titania carriersare preferred, in particular titania which has been prepared in theabsence of sulphur-containing compounds. This carrier may furthercomprise up to 50% by weight of another refractory oxide, typicallysilica or alumina. More preferably, the additional refractory oxide, ifpresent, comprises up to 20% by weight, even more preferably up to 10%by weight, of the carrier.

The preferred catalytically active metal is cobalt, although nickel,iron and ruthenium could also be used. The amount of catalyticallyactive metal present in the catalyst may vary widely. Typically, thecatalyst comprises 1-100 parts by weight of such metal per 100 parts byweight of carrier, preferably, 3-60 parts by weight, more preferably,5-40 parts by weight. The above amounts of catalytically active metalrefer to the total amount of metal in element form and can be determinedby known elemental analysis techniques. For the sake of conveniencecobalt is referred to hereinafter as the catalytically active metal, butit is emphasized that instead of or in addition to cobalt othercatalytically active metals as mentioned hereinbefore may also be used.

In addition to cobalt the catalyst may comprise one or more promotersknown to those skilled in the art. Suitable promoters include manganese,zirconium, titanium, ruthenium, platinum, vanadium, palladium and/orrhenium. The amount of promoter, if present, is typically between 0.1and 150 parts by weight, for example between 1 and 50 parts by weight,per 100 parts by weight of carrier.

Typically, the Fischer-Tropsch catalyst does not contain alkali oralkaline earth metals, apart from possible impurities introduced withstarting materials in the preparation process of the catalysts of thepresent invention. Typically, the atomic ratio of alkali or alkalineearth metals to cobalt metal is less than 0.01, preferably, less than0.005.

The Fischer-Tropsch process conditions applied in step (a) of thepresent process typically include a temperature in the range from 125 to350° C., preferably 150 to 275° C., and a pressure in the range from 5to 150 bar abs (bara). Step (a) of the present process may be operatedat the pressures conventionally applied, i.e. up to 80 bara, suitably upto 50 bara, but also higher 35 pressures can be applied.

In a preferred embodiment of the present invention step (a) comprisesreacting carbon monoxide with hydrogen at a temperature in the range offrom 125 to 350° C. and a pressure in the range from 5 to 150 bara inthe presence of a catalyst comprising cobalt on a carrier comprisingtitania. Suitably, the catalyst and process conditions in step (a) areselected such that the product obtained in this step (a) comprises inthe range of from 2 to 20% by weight of a C₁₁ to C₁₄ hydrocarbonfraction, which hydrocarbon fraction comprises in the range of from 10to 60% by weight based on total weight of this fraction of C₁₁ to C₁₄mono-olefins. This could, for instance, be achieved by using aFischer-Tropsch catalyst based on cobalt and titania at operatingtemperatures of 175 to 275° C. and operating pressures of 20 to 80 bara.

Hydrogen and carbon monoxide (synthesis gas) are typically fed to theprocess at an atomic ratio in the range from 0.5 to 4, especially from 1to 3. In a preferred embodiment, the hydrogen to carbon monoxide atomicratio is in the range from 1.5 to 2.5.

The Fischer-Tropsch reaction step (a) may be conducted using a varietyof reactor types and reaction regimes, for example a fixed bed regime, aslurry phase regime or an ebullating bed regime. It will be appreciatedthat the size of the catalyst particles may vary depending on thereaction regime they are intended for. It is within the normal skills ofthe skilled person to select the most appropriate catalyst particle sizefor a given reaction regime.

Further, it will be understood that the skilled person is capable toselect the most appropriate conditions for a specific reactorconfiguration and reaction regime. For example, the preferred gas hourlyspace velocity may depend upon the type of reaction regime that is beingapplied. Thus, if it is desired to operate the hydrocarbon synthesisprocess with a fixed bed regime, preferably the gas hourly spacevelocity (GHSV) is chosen in the range from 500 to 2500 Nl/l/h. If it isdesired to operate the hydrocarbon synthesis process with a slurry phaseregime, preferably the gas hourly space velocity is chosen in the rangefrom 1500 to 7500 Nl/l/h.

In step (b) of the present process at least one hydrocarbon fraction, ofwhich at least 95% by weight, preferably at least 98% by weight,consists of hydrocarbons containing 15 carbon atoms or more (further:C₁₅+ fraction), is separated from the hydrocarbon mixture prepared inthe preceding Fischer-Tropsch hydrocarbon synthesis step (a). Theseparation can be performed by methods known in the art. Preferably suchseparation involves a distillation treatment, notably fractionaldistillation. Conventional distillation techniques can be used.

The separation in step (b) may be effected solely by distillation, butcould also comprise a combination of fractional distillation withanother separation treatment, such as stripping or condensing. Forinstance, the hydrocarbon product from step (a) could also first beseparated into a liquid stream and a gaseous stream by passing thehydrocarbon product from step (a) through a condenser, which is suitablyoperated at similar temperature and pressure conditions as applied instep (a). The liquid stream from the condenser can then be recovered asthe C₁₅+ fraction, while the gaseous hydrocarbon stream contains thebulk of the hydrocarbons with lower carbon numbers (typically up toC₁₄). The gaseous stream is subsequently liquefied and subjected to afractional distillation treatment to recover the desired hydrocarbonfractions for further treatment

The C₁₅+ fraction will not normally contain more than 5% by weight,suitably not more than 2% by weight, of hydrocarbons containing morethan n carbon atoms with n as defined hereinafter. The C₁₅+ fractionused as the feed to step (c) may be the C₁₅+ fraction as such, but mayalso be a C₁₅-C_(n) fraction with n being an integer of at least 18,preferably at least 20, and at most 40, preferably at most 35, morepreferably at most 30. The upper limit is suitably selected such thatthe feed to the wax cracking step (d) is completely gaseous under thecracking conditions in order to avoid coke formation in the wax cracker.The remaining heavy C_(n)+ fraction could also be, in whole or in part,used as a feed to step (c) of the present process or subjected to othertreatments, such as heavy paraffin cracking treatment which results inoil products like naphtha, kerosine and gasoil. The expression “C₁₅+fraction” as used hereinafter will also include the C₁₅-C_(n) fractionas defined above.

In step (c) hydrogenation of the C₁₅+ fraction takes place. Thehydrogenation treatment is typically carried out in the presence of ahydrogenation catalyst and hydrogen at a temperature from 100 to 400°C., preferably from 100 to 300° C., more preferably, 150 to 275° C.,even more preferably 180 to 250° C. Typically, a hydrogen partialpressure is applied in the range from 10 to 250 bara, preferably from 10to 150 bara, more preferably from 10 to 50 bara, even more preferablyfrom 15 to 45 bara. Hydrogen may be supplied to the hydrogenationtreatment stage at a gas hourly space velocity in the range of from 100to 10000 Nl/l reaction zone volume/hr, more preferably from 250 to 5000Nl/l reaction zone volume/hr. The C₁₅+ fraction being treated istypically supplied to the hydrogenation treatment stage at a weighthourly space velocity in the range of from 0.1 to 5 kg/l reaction zonevolume/hr, more preferably from 0.25 to 2.5 kg/l reaction zone/hr. Theratio of hydrogen to C₁₅+ fraction may range from 100 to 5000 Nl/kg andis preferably from 250 to 3000 Nl/kg.

Hydrogenation catalysts are known to those skilled in the art and areavailable commercially, or may be prepared by methods well known in theart. Typically, the hydrogenation catalyst comprises as catalyticallyactive component one or more metals selected from Groups 6, 8, 9 and 10of the Periodic Table of Elements, in particular one or more metalsselected from molybdenum, tungsten, cobalt, nickel, ruthenium, iridium,osmium, platinum and palladium. Preferably, the catalyst comprises oneor more metals selected from nickel, platinum and palladium as thecatalytically active component. A particularly suitable catalystcomprises nickel as a catalytically active component.

Catalysts for use in the hydrogenation treatment stage typicallycomprise a refractory metal oxide or silicate as a carrier. Suitablecarrier materials include silica, alumina, silica-alumina, zirconia,titania and mixtures thereof. Preferred carrier materials for inclusionin the catalyst for use in the process of this invention are silica,alumina, silica-alumina, and diatomaceous earth (kieselguhr).

The catalyst may comprise the catalytically active component in anamount of from 0.05 to 80 parts by weight calculated as element,preferably from 0.1 to 70 parts by weight, per 100 parts by weight ofcarrier material. The amount of catalytically active metal present inthe catalyst will vary according to the specific metal concerned. Oneparticularly suitable catalyst for use in the hydrogenation treatmentstage comprises nickel in an amount in the range of from 30 to 70 partsby weight (calculated as element) per 100 parts by weight of carriermaterial. A second particularly suitable catalyst comprises platinum inan amount in the range of from 0.05 to 2.0 parts by weight per 100 partsby weight of carrier material.

In subsequent step (d) mild thermal cracking of the hydrogenatedhydrocarbon fraction obtained in step (c) takes place. This mild thermalcracking can be carried out by ways known in the art. In one preferredembodiment the mild thermal cracking step (d) is carried out in thepresence of steam. Such treatment is, for instance, described in theaforementioned U.S. Pat. No. 4,579,986, which is incorporated byreference herein. A suitable mild thermal cracking treatment involvescracking the hydrogenated hydrocarbon fraction at a temperature of from450 to 675° C., preferably 480 to 600° C., a pressure of from 1 to 50bara, preferably 1 to 10 bara and more preferably 1 to 5 bara, and aresidence time of 0.5 to 20 seconds, preferably 1 to 10 seconds. Thethermal cracking can be carried out with or without diluent. Suitablediluents include steam and inert gases, of which steam is preferred. Ifused, steam is typically used in an amount of up to 40% by weight (basedon hydrocarbon feed), preferably 3 to 30% by weight. As indicated, aninert gas may also be used as diluent. An inert gas in this connectionis a gas, which does not interfere with the cracking reactions bydecomposition and/or reacting with the hydrocarbon reactants andcracking products. Examples of suitable inert gases include nitrogen andnoble gases like helium and argon. It was found that using a diluent hasa positive impact on the amount of by-products formed.

In a preferred embodiment the mild thermal cracking treatment comprisesthe stages of:

(d1) combining the diluent and the hydrogenated hydrocarbon fraction inan evaporator, and

(d2) thermally cracking the evaporated hydrocarbon fraction.

The evaporator is usually operated at a temperature sufficiently high toevaporate the hydrogenated hydrocarbon stream. This will normally be atleast 350° C., suitably at least 400° C., while the maximum temperaturewill usually not exceed 600° C., suitably 500° C., in order to avoidexcessive cracking. The actual cracking in stage (d2) typically takesplace at a temperature of from 450 to 650° C., suitably 480 to 600° C.,a pressure of at least 1 bara and usually not more than 300 bara,suitably from 1 to 10 bara, more suitably 1 to 5 bara, a residence timeof 0.5 to 20 seconds, suitably 1 to 10 seconds in the presence of thediluent.

As will be discussed hereinafter, subsequent separation step (e) mayyield a heavy C_(m)+ hydrocarbon stream, which can be at least partlyrecycled to cracking step (d), either directly or via hydrogenation step(c). In such mode of operation thermal cracking step (d) is suitablycarried out under such conditions that the conversion of hydrocarbonsper pass is in the range of from 10 to 50% by weight, preferably 10 to35% by weight and more preferably 15 to 30% by weight, based on totalweight of hydrocarbons passed through the thermal cracking reactor inthat pass.

In subsequent step (e) the desired mixture comprising the C₅+ linearolefins is separated from the cracked product. In principle anyseparation technique suitable for separating the C₅+ hydrocarbon mixturefrom the cracked product can be used. This could involve short pathdistillation techniques like separation with a wiped film evaporator,stripping techniques and fractional distillation at atmospheric orreduced pressure. For the purpose of the present invention oneparticularly suitable separation method comprises the stages of:

(e1) cooling the cracked product and separating from the cooled crackedproduct the liquid cracked product containing the C₅+ hydrocarbons, and

(e2) separating from the liquid cracked product the mixture comprisingthe C₅-C_(m) linear olefins.

Typically cooling and first separation stage (e1) take place in agas/liquid separator. The hot cracked product is first cooled to atemperature at which the desired C₅+ hydrocarbons become liquid and thegaseous C₁ to C₄ products as well as any diluent used in the crackingtreatment can be removed as gases. It will be understood that a smallamount of C₅+ hydrocarbons will end up in the gaseous stream, while asmall portion of the C₄-hydrocarbons will end up in the liquid stream.It will be appreciated that the exact temperature applied depends on thepressure applied. The liquid stream recovered from the gas/liquidseparator contains the desired C₅+ hydrocarbons and is fed to subsequentseparation stage (e2), where the C₅-C_(m) hydrocarbon stream containingthe mixture of C₅-C_(m) linear olefins is separated. This stage cansuitably be conducted in a stripper, optionally using a stripping gaslike steam, nitrogen, helium or argon. The C₅-C_(m) hydrocarbon streamcontaining the mixture of C₅-C_(m) linear olefins is recovered as thetop fraction. In a further separation stage (e3) the C_(m)+ bottomfraction is suitably at least partly recycled to hydrogenation step (c)and/or to cracking step (d).

The mixture comprising C₅+ linear olefins as obtained by the processdescribed hereinbefore typically comprises from 20 to 50% by weight ofC₁₁ to C₁₄ linear α-olefins and from 40 to 75% by weight of C₅ to C₁₀linear α-olefins and hence is a very suitable feedstock for preparinglinear detergent and plasticizer alcohols in a hydroformylationreaction.

Accordingly, in a second aspect the present invention relates to aprocess for the preparation of linear alcohols by reacting anolefin-containing feed with carbon monoxide and hydrogen in the presenceof an effective amount of hydroformylation catalyst underhydroformylation conditions, wherein the olefin-containing feed is atleast partly based on the mixture comprising C₅+ linear olefins obtainedby the process as described hereinbefore.

A very suitable process in this connection is a process, wherein theolefin-containing feed is obtained by subjecting to a fractionationtreatment:

(a) a first hydrocarbon stream derived from reacting carbon monoxide andhydrogen in the presence of an effective amount of Fischer-Tropschcatalyst under Fischer-Tropsch reaction conditions, and

(b) a second hydrocarbon stream consisting of the mixture comprising C₅+linear olefins obtained by the process as described hereinbefore.

The weight ratio of the first hydrocarbon stream to the secondhydrocarbon stream may vary within broad limits, but suitably is in therange of from 0.1:1 to 30:1, preferably 1:1 to 30:1 and more preferably5:1 to 25:1.

The fractionation treatment suitably corresponds with separation step(b) of the process according to the first aspect of the presentinvention as described hereinbefore.

The first hydrocarbon stream is the product of a Fischer-Tropschhydrocarbon synthesis reaction, suitably the entire C₄+ productrecovered from a Fischer-Tropsch hydrocarbon synthesis reaction. Thisreaction, its conditions and ways of operation have been extensivelydiscussed hereinbefore. Suitably, the first hydrocarbon stream comprisesfrom 2 to 20% by weight, more suitably 3 to 10% by weight, of C₁₁ to C₁₄hydrocarbons. Of these C₁₁ to C₁₄ hydrocarbons 10 to 60% by weight,suitably 15 to 50% by weight, consists of C₁₁ to C₁₄ linearmono-olefins.

The second hydrocarbon stream consists for at least 95% by weight,preferably at least 98% by weight, of hydrocarbons comprising 5 or morecarbon atoms and typically comprises from 20 to 50% by weight of C₁₁ toC₁₄ linear α-olefins, while levels of 30% by weight or more and even 35%by weight or more are also achievable. The amount of C₅ to C₁₀ linearα-olefins in the second hydrocarbon stream typically ranges from 40 to75% by weight. The balance up to 100% by weight consists of hydrocarbonsother than the olefins mentioned, such as C₄ hydrocarbons and thecorresponding C₅+ linear alkanes, iso-alkanes, iso-olefins, internalolefins and dienes.

FIG. 1 shows a simplified flow scheme of an exemplary process accordingto the second aspect of the present invention

In this FIG. 1 a Fischer-Tropsch C₄+ hydrocarbon product stream 6obtained in a Fischer-Tropsch hydrocarbon synthesis process (not shown)is passed into fractionation column 1. A C₄-C₅ fraction 7, a C₆-C₁₀fraction 8, a C₁₁-C₁₄ fraction 9, a C₁₅-C₃₀ fraction 10 and a C₃₀+fraction 11 are recovered. The C₆-C₁₀ fraction 8 and the C₁₁-C₁₄fraction 9 are passed into hydroformylation unit 5, where they areconverted into respectively plasticizer alcohols 17 and detergentalcohols 18. The C₃₀+ fraction 11 can be passed into heavy paraffincracker (not shown) to be converted in e.g. middle distillates likenaphtha and kerosine. The C₁₅₋₃₀ fraction 10 is passed intohydrogenation unit 2, resulting in hydrogenated fraction 12, which issubsequently cracked in mild thermal cracking unit 3. The crackedproduct 13 is fed to fractionation unit 4, from which a C₄− fraction 15,a C₅-C₁₄ fraction 16 and a C₁₅+ fraction 14 are recovered. The latter isrecycled to cracking unit 3, while the C₅-C₁₄ fraction 16 is combinedwith Fischer-Tropsch C₄+ hydrocarbon product stream 6 and passed intofractionation column 1.

The invention is further illustrated by the following examples withoutlimiting the invention to these specific embodiments.

EXAMPLE 1

Two commercially available hydrogenated Fischer-Tropsch reactor products(available under the trade marks SX-30 and SX-50) were combined in aweight ratio SX-50:SX-30 of 70:30 and subsequently 5% by weight (basedon the total weight of SX-30 plus SX-50) of hexadecane was added inorder to simulate a hydrogenated C₁₆+ Fischer-Tropsch feed for the waxcracker. The composition of this feed is shown in Table 1.

TABLE 1 Feed compositions after hydrogenation C-fraction (% by weight)C₁₄− paraffin 0.0 C₁₅-C₂₀ paraffin 32.9 C₂₁-C₂₅ paraffin 26.4 C₂₆-C₃₀paraffin 35.3 C₃₁+ paraffin 5.4

For the actual cracking reaction a AISI 310 reactor tube (length 30 cm,volume 10 ml) was used. Accordingly, the hydrogenated fraction wassubsequently combined at a feed rate of 12 grams per hour with recycledC₁₅-C₂₀ fraction recovered from the cracked product at a recycle ratio(i.e. weight ratio of recycled fraction to fresh hydrogenated fraction)of 3.2. The combined paraffinic stream was dosed at 70° C. from a heatedstorage vessel into an evaporator where it was combined with helium at amole ratio helium to hydrocarbon of 1. The temperature in the evaporatorwas approximately 400° C. The evaporated stream was subsequently passedinto the cracking zone where cracking took place at a temperature of560° C. and a pressure of 3 bara at a residence time of 4 seconds. Thecracked product was subsequently separated into a gaseous fraction(helium and C₁-C₄ hydrocarbons), a liquid cracked product C₅-C₁₄fraction and a liquid C₁₅-C₂₀ product, which was recycled to be combinedwith freshly hydrogenated feed prior to entering the evaporator. Thecomposition of the cracked product C₁-C₁₄ is indicated in Table 2.

TABLE 2 Composition of cracked product Weight % ¹⁾ Fraction C₁-C₄ 37(par + olef)  C₅-C₁₄ 63 (par + olef) Mono-olefin C₁₁-C₁₂ 9.7 C₁₃-C₁₄ 9.2¹⁾ based on total weight of C₁-C₁₄ formed

EXAMPLE 2

The liquid cracked product C₅-C₁₄ fraction obtained in Example 1 wasfractionated using a 15 tray packed Fischer packed distillation columnat a reflux ratio of 25. The C₁₁/C₁₂ fraction and the C₁₃/C₁₄ fractionobtained through this fractionation were subjected to a hydroformylationtreatment to produce the corresponding alcohols. The compositions of theboth fractions are given in Table 3.

Hydroformylation of both fractions was carried by charging a 1.5 literautoclave with 565 grams of feedstock consisting of 53% by weight of theC₁₁/C₁₂ fraction or the C₁₃/C₁₄ fraction, 34% by weight of iso-octane(as diluent), 1% by weight of n-decane or tetradecane (as internalstandard for respectively the C₁₁/C₁₂ fraction and the C₁₃/C₁₄ fraction)and 12% by weight of 2-ethylhexanol in which KOH and thehydroformylation catalyst were dissolved. The hydroformylation catalystwas based on cobalt octanoate as cobalt precursor and9-eicosyl-9-phosphabicyclononane as the ligand and these were added insuch amount that the amount of catalyst was 0.25% by weight based ontotal feedstock added and the ligand/cobalt molar ratio was 1.2. Theamount of KOH present in the 2-ethylhexanol was such that the K/Co molarratio amounted to 0.4. Hydroformylation was subsequently carried out at192° C. and 70 bara synthesis gas (H₂/CO molar ratio=2). The reactiontime was 3 hours. The conversion achieved was >98.5%.

The crude alcohol products obtained were successively subjected to asingle stage evaporative distillation (Rotavap operation at 100 mbaraand bath temperature of 80-220° C.), saponification through the additionof NaHB₄ at 50-90° C., two water washing treatments at 80-90° C. toremove the inorganic salts formed and a distillative treatment atreduced pressure to remove the light and heavy products (“topping andtailing” treatment).

The composition of the alcohol products thus obtained is indicated inTable 3.

TABLE 3 Hydroformylation of cracked product Cracked Alcohol CrackedAlcohol C_(11/12) C_(12/13) C_(13/14) C_(14/15) C-fraction (% w) (% w)(% w) (% w) C10 alkenes (all) 3.3 n-alkane <0.1 linear alcohol <0.1 C111-alkene 43.9 other alkenes¹⁾ 3.9 n-alkane 0.2 <0.1 linear alcohol <0.1<0.1 C12 1-alkene 40.5 1.0 other alkenes¹⁾ 4.6 0.3 n-alkane 1.6 <0.1<0.1 <0.1 linear alcohol 41.3 <0.1 branched alcohol 7.1 C13 1-alkene 1.044.5 other alkenes¹⁾ 0.7 4.7 n-alkane 0.2 1.4 <0.1 linear alcohol 34.40.8 branched alcohol 16.0 C14 1-alkene 40.3 other alkenes¹⁾ 4.5 n-alkane1.4 <0.1 linear alcohol 42.2 branched alcohol 0.9 13.3 C15 1-alkene 0.7other alkenes¹⁾ 0.5 n-alkane 0.2 <0.1 linear alcohol 30.4 branchedalcohol 12.3 C16 branched alcohol 0.8 ¹⁾other alkenes include dienes,branched alkenes and internal alkenes

We claim:
 1. A process for the preparation of a mixture comprising C₅+linear olefins, which process comprises the steps of (a) reacting carbonmonoxide and hydrogen in the presence of a Fischer-Tropsch catalystunder Fischer-Tropsch reaction conditions thereby producing ahydrocarbon mixture; (b) separating, from the hydrocarbon mixture, atleast one hydrocarbon fraction, of which at least 95% by weight consistsof hydrocarbons containing 15 carbon atoms or more; (c) contacting thethus-separated hydrocarbon fraction with hydrogen in the presence of ahydrogenation catalyst under hydrogenation conditions thereby producinga hydrogenated hydrocarbon fraction; (d) subjecting said hydrogenatedhydrocarbon fraction to a mild thermal cracking treatment therebyproducing a cracked product; and (e) separating, from the crackedproduct, a mixture comprising C₅+ linear olefins.
 2. The process ofclaim 1 wherein the mixture comprising C₅+ linear olefins is a mixturecomprising C₅ to C_(m) linear olefins with m representing an integer offrom 10 to
 20. 3. The process of claim 1 wherein the mild thermalcracking step (d) is carried out in the presence of a diluent.
 4. Theprocess of claim 3 wherein the diluent is steam.
 5. The process of claim1 wherein separation step (e) comprises the steps of: (e1) cooling thecracked product and separating from the cooled cracked product theliquid cracked product containing the C₅+ hydrocarbons, and (e2)separating from the liquid cracked product the mixture comprising theC₅-C_(m) linear olefins.
 6. The process of claim 5 wherein theseparation step (e) comprises a further step: (e3) recycling at leastpart of the C_(m)+ bottom fraction from separation step to cracking step(d) and/or hydrogenation step (c).
 7. The process of claim 1 wherein thethermal cracking step (d) is carried out under conditions in a thermalcracking reactor that the conversion of hydrocarbons per pass is in therange of from 10 to 50% by weight based on total weight of hydrocarbonspassed through the thermal cracking reactor in that pass.